With a trend of focusing on environmental issues, systems that convert clean energy such as sunlight or wind power into electricity and store the clean energy as electric energy are being intensively developed. Known examples of such storage devices include a lithium ion secondary battery (LIB: lithium ion battery), an electric double layer capacitor (EDLC), and a lithium ion capacitor. Recently, capacitors such as an EDLC and a lithium ion capacitor have been attracting attention because capacitors are excellent in terms of instant charge-discharge characteristics, high-output characteristics, and ease of handling.
Such a capacitor includes a first electrode and a second electrode that individually contain electrode active materials, and an electrolyte. The electrode active materials are carbon materials and the like. For example, an EDLC employs activated carbon as an electrode active material. A lithium ion capacitor employs, as a positive-electrode active material, activated carbon and, as a negative-electrode active material, a carbon material that can be intercalated and deintercalated by lithium ions (Patent Literature 1). In order to increase energy density (or capacitance), it has been studied that a graphite in which a plurality of amorphous portions having an average area of 1.5 nm2 or more are dispersed over (002) planes is used as the positive-electrode active material of a capacitor (Patent Literature 2). It has also been studied that carbon particles having nano-pores are used as the electrode active material of an EDLC (Patent Literature 3).
In general, electrolytes of capacitors are organic solvent solutions (organic electrolytes) containing supporting electrolytes. Examples of organic solvents of the electrolytes include ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) (for example, refer to Patent Literature 2).